A radio intended for two-way communication generally operates with either an external fixed rod or retractable antenna, or with an internal antenna. The fixed rod type of antenna has a predetermined length. Whilst such antenna as can be relatively short, they are not conducive to a compact design nor are they particularly suitable for a radio intended to be carried in a pocket or other receptacle offering restricted space. On the other hand, retractable antennas are convenient for this purpose because they can be folded away when the radio is not in use. Retractable antennas are commonly of the telescopic tube type, although retractable fixed length antennas are also known.
Generally, there has been a trend to using more compact antennas such as internal antennas or retractable antennas. This is due to users of radios desiring the radios to be as compact as possible. Retractable or foldable antennae are mechanically coupled to the radio housing by a moveable joint, for example a rotating joint or a make or break connector comprising complementary colletts as described in British Patent Application 2 257 836. A problem with such moveable joints is that they do not provide good radio frequency coupling to radio circuitry disposed in the housing and are a source of high losses, especially at microwave frequencies.
This is a particular problem in applications where the received radio signal is of a very low power level, for example satellite receivers. In a GPS satellite receiver a Low Noise Amplifier (LNA) has been included in the antenna housing before the coupling to the radio housing. An example of such an antenna and LNA is a dielectric patch antenna package manufactured by FDK Corporation Model No. DA-IC05.
It is noted that the term "elongate antenna element" as used herein encompasses for example a rod type antenna or a coil type antenna a having a generally elongate configuration. Also the term "helical" is not restricted to a helix having a uniform diameter but is intended to include a coil having a progressively widening diameter, viz. a spiral configuration, and multi-filar configurations.